Electrically conductive rear view mirror

ABSTRACT

An electrically conductive product generally comprised of a substrate or web coated with or impregnated with an electrically conductive thermally stable material which includes a special polymeric binder and conductive particles to produce the final conductive sheet.

This is a division, of application Ser. No. 364,749, filed May 29, 1973now U.S. Pat. No. 3,948,811.

BACKGROUND OF THE INVENTION

This invention broadly relates to a new electrically conductive sheetmaterial and composition. More particularly this invention relates toelectrically conductive sheet material which is highly useful forgenerating heat at low voltages, for example, it finds particularutility in windshield wiper applications where the sheet in contact withthe wiper blade may be heated to free the wiper when it is frozen inposition during winter weather; or, it may be used in contact with theoutside rear view mirror to heat same and remove ice or snow.

The state of the art is indicated by the following references: U.S. Pat.Nos. 3,099,578; 3,573,230; 2,983.624; 3,025,185; 2,968,649; 3,132,124;3,051,677; DuPont Viton Bulletin No. 16 "Solution Coatings of Viton" byJ. M. Bowman; and, DuPont's "The Engineering Properties of VitonFluoroelastomer"; and, Acheson Industries Inc. product data sheet for"Electrodag 415". The disclosures of these references are herebyincorporated herein by reference.

Accordingly a main object of this invention is to provide a novelelectrically conductive sheet type material.

Another object of the present invention is to provide an electricallyconductive rear view mirror having a substrate with an electricallyconductive coating composition thereon.

Another object of the present invention is to provide a new sheetmaterial or product which is uniquely suitable in windshield wiperapplications for applying heat to the wiper blade to free same underadverse weather conditions as occur in winter from ice, snow, orfreezing rain, and thus providing a considerably safer windshield wipersystem.

Another object of the present invention is to provide a new flexiblepressure sensitive adhesive tape sheet which is electrically conductiveas herein described.

Another object of the present invention is to provide a new electricallyconductive sheet product which incorporates a special combination ofthermally stable fluoroelastomer polymer resin component and aconductive pigment component.

Another object of the present invention is to provide a novel heatgenerating product, which may be stamped out in decal form or in variouspatterned shapes.

Another object of the present invention is to provide a novelelectrically conductive sheet material or composition which is uniquelycapable of having electrical contacts soldered to its surface.

Other objects, features, and advantages of the present invention willbecome apparent from the subsequent description and the appended claimstaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an electrically conductive sheet type material inaccordance with the invention;

FIG. 2 is a top view of FIG. 1;

FIGS. 3 and 4 illustrate a windshield wiper application of the inventionherein, with FIG. 4 being a cross sectional view along the line 4--4 inFIG. 3;

FIG. 5 illustrates an electrically conductive sheet material inaccordance with the invention herein which is made in the form of apressure sensitive adhesive tape.

FIG. 6 is a cross-sectional view of a rear view mirror application ofthe invention herein.

SUMMARY OF THE INVENTION

This invention of an electrically conductive sheet or heat generatingsheet is capable of providing low resistance elements useful forgenerating heat at low voltages, such as in the range of 1-40 volts asused in battery powered devices or automotive, industrial andrecreational vehicles. Also the sheet material discovered and disclosedherein is suitable and adaptable for heat generation at electricalvoltages normally used for household electrical current. The sheetmaterial herein may satisfactorily be formed on and supported by a wovenor nonwoven glass web or felted fabric (from which, for the highesttemperature stability, it is desirable to burn off or remove the bondingand sizing material employed in its manufacture).

By providing the supporting web, the sheets formed are characterized byflexibility, strength, and mechanical stability not possessed by earlierknown coatings or films. The self-supporting sheets of this inventioncan be fabricated and insulated for use as such or bonded to substrates.

Preferred substrates for use herein are web members available inthicknesses from about 1 mil up to approximately 20 mils. By employingthe special compositions as are disclosed herein comprising heatresistant polymeric binder and conductive particles, it is possible toposition or entrap controllable amounts of conductive particles on theweb or within voids and interstices of the web, in such a way that theelectrical resistance can be controlled, and thermally stable,mechanically strong, flexible, conductive webs or sheets may thus beformed.

The electrical resistance and the heat generating capacity is a functionof the nature and the composition of the web, the pigment-binder ratio,the extent to which the sheet or web matrix is saturated, and thethickness of the web or sheet.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate a preferred form of the conductive sheetprepared in accordance with the invention designated 10. The sheet 10 ismade from a nonwoven glass web which has previously had bonding orsizing agents removed therefrom. A special composition 16 is impregnatedinto the web with the composition 16 being made from a thermally stableheat resistant polymeric binder and conductive particles. The materialof the conductive sheet 16 may be uniquely receptive to havingelectrical leads or electrical contacts 17 and 19 soldered thereto aswill be explained in more detail hereinbelow.

FIGS. 3 and 4 illustrate a windshield wiper device in accordance withthe invention wherein the electrical properties of the conductive sheetherein described are utilized in conjunction with a windshield wiper 300for the purpose of transferring electrically generated heat to the wiper300 to heat same to free the wiper from an ice-bound condition duringwinter weather. The wiper 300 is suitably constructed in a known fashionutilizing for example a metal reinforcing member 302 for supporting aflexible rubber portion 304 with the wiper being supported through thereinforcing member 302 to the oscillating arm (not shown) which movesthe wiper through its back and forth motion during use. As will be notedthe wiper contains electrically conductive sheet 316 applied theretowhich sheet 316 is formed in accordance with the invention herein. Thesheet 316 is electrically conductive and is adapted to be connected withlead wires which operate to introduce electrical current to the sheet316 for the purpose of heating the wiper in order to free the wiper froman ice-bound condition. The sheet 316 may be associated with or appliedto the wiper 300 by any suitable means such as through the use of anadhesive or the like.

It should also be understood that the electrically conductive sheet 316may also suitably be deposited in strip form to a flat surface on whichthe rubber wiper blade 300 comes to rest when the wiper is in its parkedposition. By this technique the flat surface of the electricalyconductive sheet 316 would be in contact with the wiper only when thewiper was in its parked position, however, electrically conductive heatin this embodiment is still applied to the electrically conductive sheet316 to free the wiper from an ice-bound condition so that the wipercould then move upwardly into its oscillating mode of operation forwiping the windshield of the vehicle.

FIG. 5 illustrates another embodiment of the invention herein whereinthe electrically conductive sheet 16 has a pressure sensitive adhesive(PSA) coating or layer 18 applied to at least one side thereof. Numeroussuitable pressure sensitive adhesives may be used for this purpose witha suitable pressure sensitive adhesive being that described in U.S. Pat.No. 2,884,126, or Monsanto Co., Gelva multi-polymer solution, RA-1753,pressure sensitive adhesive. The pressure sensitive adhesive preferredis a high temperature resistant acrylic or silicone type. Said patentalso describes different techniques for applying the pressure sensitiveadhesive 18 to backing support such as the electrically conductive sheet16. Application of the pressure sensitive adhesive is suitably carriedout by knife coating, roller coating, dry transfer coating, or semi-wettransfer coating.

Electrically conductive sheet 16, as shown in FIG. 1 may also be appliedto one surface of a double adhesive sided pressure sensitive adhesivetape to obtain a conductor which may be applied to any substrate surfacein a convenient fashion. The electrical contacts may then be soldered atthe desired points. This construction may then be insulated by coveringit with another pressure sensitive adhesive tape. Also by applyingpressure sensitive adhesive tape to either side of the sheet 16insulated sheet heating elements or resistive members can be obtained.Electrical resistance may be adjusted to desired level by altering theconductive particles, the pigment-binder ratio, and the shape of theconductor.

FIG. 6 illustrates an electrically conductive rear view mirror 600 inaccordance with the present invention, in which the back side of themirror glass portion 602 is coated with electrically conductive coatingcomposition 616. Electrical leads or contacts 617 and 619 may besoldered to the coating 616 in order to introduce electrical current tosaid coating for the purpose of heating the mirror glass portion 602 andthereby melting any ice or snow.

The fluoroelastomer and the silicone elastomer materials used hereinshould be present within the broad range of about 5% to about 90% byweight of the coating solids and preferably from about 5% to about 80%by weight. The fluoroelastomer or the silicone elastomer materialsprovide the special function of endowing the coating with the highthermal stability, that is, stability at temperatures of about 400°-700°F and higher. The fluoroelastomer material used in the invention shouldbe one which provides this high thermal stability while at the same timepossessing the property of being able to operate as an effective binder.Particularly suitable materials for use as the fluoroelastomer hereinmay be described as vinyl/olefinic fluoroelastiomeric polymers,vinyl-fluorocarbon elastomeric copolymers, vinylidene/fluoro-olefinicelastomeric polymers, and C₂ -C₄ olefinic/fluorocarbon elastomericpolymers. A preferred material is the vinylidenefluoride/hexafluoropropylene copolymer fluoroelastomer. Commerciallyavailable fluoroelastomers which may be used are Viton A or B, (DuPonttrademark) and Fluorel FC-2170 or KF-2140 (3 M Company trademark). Stillfurther fluoroelastomers which may be used herein are described in U.S.Pat. Nos. 2,968,649, 3,051,677, and 3,172,124, the disclosures of whichare incorporated herein by reference. A commercially available siliconeelastomer for use herein is Union Carbide Silicone product KS-1315. Thepreferred silicone elastomeric resin should be high temperatureresistant and have a stable pot life in admixture with other ingredientsof the composition.

The conductive particles or pigment for use in the sheet or coatingshould be present within the broad range of about 10% to about 95% byweight of the total solids of the coating and preferably this rangeshould be from about 20% to about 95% by weight. Suitable particles orpigments for use in the coating are various finely particulated pigmentssuch as silver particles, copper particles, graphite, conductive carbon,silver coated particles, noble metals and alloys thereof. Silver orsilver coated particles are preferred for the formation of highlyconductive sheets. With conductive carbon the percentage range wouldnormally be 20-30% and with silver it would normally be 85-95%.Compositions incorporating metallic conductive particles are wetted byconventional soldering alloys.

The silver pigment preferably employed is a silver powder having forexample a mesh size of about 90% through 325 mesh; and an apparentdensity of about 16-32 grams per cubic inch (Scott volumeter).

Impregnation of the web with conductive material is achieved by the useof paint-like compositions which include the pigment particles and thebinder in suitable solvents. The fluid or pasty composition is appliedto the web and then dried and cured.

Solvents which are suitable for the compositions are those which arecompatible with and also act as solvents for the fluoroelastomerconstitutent.

The percentage of metallic conductive pigment particles required in thiscoating composition should be above the minimum levels as generallydescribed above in order to obtain coatings to which solder will adherereadily and to provide suitable conductivity. This is a function to someextent of the particle size and shape of the silver particles, and theparticular fluoroelastomer material being used. When the conductiveparticle content, such as the silver particles, is over 95% the coatingslose integrity and adhesion.

The fluid carrier material used for forming solutions or dispersions ofthe coating material may satisfactorily be selected from any number ofdifferent compatible solvents and blends thereof such as methylethylketone, acetone, various other ketone or ester type solvents,dimethylformamide, and numerous other organic solvent materials orwater. The coating compositions may suitably have total solids contentsbetween about 1% and 80% by weight. The carrier fluid is driven off orevaporated to leave the dry coating bonded within or on the web.

A curing agent may optionally be used in the coating. For this purposevarious amino type curing agents and peroxide curing agents and the likemay be used.

The web may be any flexible porous inorganic fibrous material. Amongcommercially available web materials are Nomex (DuPont) and Craneglas(Crane & Co. Inc.).

resistance character of coatings such as may be applied to a web isevaluated by forming dried coatings under uniform conditions, upon whichthe electrical resistance measurements can be made. Measurements hereinhave been made on a coating after curing for ten minutes at 300° F.

The electrical resistance was measured using a special fixture whichgave data in ohms per square. The resistance measurement was made with asuitable precision milliohmmeter.

The following are example coatings with which a glass type paper web maybe impregnated to form sheets suitable for generating heat with appliedvoltage.

    ______________________________________                                                       Example   Exaple    Example                                                   1         2         3                                          ______________________________________                                        Silver Powder  90.91     93.35     71.49                                      *Fluoroelastomer #1                                                                          9.09      6.65      28.51                                      Methyl Ethyl Ketone                                                                          62.33     23.34     240.42                                     Resistance ohms per                                                           square at 1 mil                                                               thickness.     0.19      .152      1.7                                                       Example   Example   Example                                                   4         5         6                                          ______________________________________                                        Silver Powder  93.35     93.35     90.32                                      *Fluoroelastomer #2      6.65                                                 #3             6.65                9.68                                       Methyl Ethyl Ketone                                                                          66.68     66.68     77.42                                      Resistance ohms per                                                           square at 1 mil                                                               thickness      .3        .15       .23                                        ______________________________________                                         *Note                                                                         #1 = Viton B (DuPont)                                                         #2 = M's FC-2170                                                              #3 = 3 M's KF-2140                                                       

EXAMPLE 7______________________________________ Example7______________________________________Silver Powder 90.50Union CarbideSilicone Rubber(KS 1315 at 35% Solids 9.50Xylol 87.00Lubrizol 500R(Dicumyl Peroxide) 0.68Resistance ohms persquare at 1 mil thickness0.20______________________________________ Viton B(Fluoroelastomer -1)is a terpolymer of vinylidene fluoride - hexafluoropropylene and a smallamount of tetrafluoroethylene. The fluoroelastomer material 3M FC-2170(Fluoroelastomer -2) is a fluoroelastomer polymer material made byMinnesota Mining & Manufacturing Company having the following physicalproperties:______________________________________Specific Gravity1.81Appearance opaque, off-whiteSolubility Esters and KetonesMooneyViscosity - Large Rotor at 212° F4 Minute Reading60______________________________________

The fluoroelastomer material 3M KF-2140 (Fluoroelastomer -3) is afluoroelastomer polymer material made by Minnesota Mining andManufacturing Company having the following physicalproperties:______________________________________Specific Gravity1.81Appearance opaque, off-whiteSolubility Esters and KetonesMooneyViscosity - Large Rotor at 212° F4 Minute Reading110______________________________________

EXAMPLE NO. 8

Using the material from Example No. 1 a 5 mil sample of glass paper(Craneglas) from Crane Co. was coated by passing the web through a bathof the Example No. 1 coating and then through a squeegee to a heatingoven.

This is a typical "spreader" squeegee operation as is common in the tapeor paper industry. Impregnation occurred and the sheet was cured for anequivalent time of 10 minutes at 300° F.

With 5 mil paper the average resistance was 0.008 ohms per square.

EXAMPLE NO. 9

The procedure of Example 8 was repeated impregnating 3 mil thick glasspaper using a reduction of one part of solvent (MEK) to four parts ofExample No. 1 material. The average resistance of the sheet produced was0.043 ohms per square.

While it will be apparent that the preferred embodiments of theinvention disclosed are well calculated to fulfill the objects abovestated, it will be appreciated that the invention is susceptible tomodification, variation, and change without departing from the properscope or fair meaning of the subjoined claims.

What is claimed is:
 1. An electrically conductive rear view mirrorhaving a substrate with an electrically conductive coating compositionthereon,said composition comprising in weight percent, about 5% to about90% of a heat stable fluoroelastomer polymer about 10% to about 95% ofconductive particles, said conductive particles are a finelyparticulated material selected from at least one material from the groupconsisting of silver, copper, noble metals and alloys thereof, graphite,or conductive carbon, said composition being for the purpose of heatingthe mirror.
 2. The invention of claim 1 wherein, said conductiveparticles are substantially of graphite or conductive carbon.
 3. Theinvention of claim 1 wherein,said conductive particles are substantiallygraphite particles.
 4. The invention of claim 1 wherein,saidfluoroelastomer polymer is comprised at least partially of avinyl/olefinic fluoroelastomeric polymer material.
 5. The invention ofclaim 1 wherein,said electrically conductive coating composition isapplied to the back side of said substrate.
 6. An electricallyconductive rear view mirror having a substrate with an electricallyconductive sheet thereon in adhesive tape form,said sheet including anelectrically conductive composition comprised of, in weightpercent,about 5% to about 90% of heat stable fluoroelastomer polymer,and about 10% to about 95% of conductive particles, said conductiveparticles being a finely particulated material selected from at leastone material from the group consisting of silver, copper, noble metalsand alloys thereof, graphite, or conductive carbon, said compositionbeing for the purpose of heating the mirror.
 7. In a rear view mirrorapparatus, the improvement comprising,an electrically conductive sheetmaterial in adhesive tape form, said sheet having an adhesive coatingapplied to at least one side thereof, said sheet including anelectrically conductive composition comprised of, in weightpercent,about 5% to about 90% of heat stable fluoroelastomer polymer,and about 10% to about 95% of conductive particles, said conductiveparticles being a finely particulated material selected from at leastone material from the group consisting of silver, copper, noble metalsand alloys thereof, graphite, or conductive carbon, said compositionbeing for the purpose of heating the mirror.